1. Field of the Invention
The present invention relates broadly to holographic memory systems, and more particularly to methods and systems for holographic data retrieval.
2. Related Art
Developers of information storage devices continue to seek increased storage capacity. As part of this development, memory systems employing holographic optical techniques, referred to as holographic memory systems, have been suggested as alternatives to conventional memory devices.
Holographic memory systems may read/write data to/from a photosensitive storage medium. When storing data, holographic memory system often record the data by storing a hologram of a 2-dimension array, commonly referred to as a “page,” where each element of the 2-D array is a single data bit. This type of system is often referred to as “page-wise” memory system. Holographic memory systems may store the holograms as a pattern of varying refractive index and/or absorption imprinted into the storage medium.
Holographic systems may perform a data write (also referred to as a data record or data store operation, simply “write” operation herein) by combining two coherent light beams, such as laser beams, at a particular point within the storage medium. Specifically, a data-encoded light beam is combined with a reference light beam to create an interference pattern in the photosensitive storage medium. The interference pattern induces material alterations in the storage medium to form a hologram.
Holographically-stored data may be retrieved from the holographic memory system by performing a read (or reconstruction) of the stored data. The read operation may be performed by projecting a reconstruction or probe beam into the storage medium at the same angle, wavelength, phase, position, etc., as the reference beam used to record the data, or compensated equivalents thereof. The hologram and the reference beam interact to reconstruct the data beam.
The reconstructed data beam may then be detected by a power-sensitive camera and processed for delivery to an output device. This detection mechanism may have several limitations. First, since hologram diffraction efficiency is driven to the lowest possible level in order to maximize the number of pages that may be stored, the read signals may be weak and require long exposure times to detect. Secondly, the laser light used to perform the read-out may be necessarily coherent, thus optical noise sources such as scatter and ISI (intersymbol interference, or pixel-to-pixel crosstalk from blur) may mix coherently with the desired optical signal, reducing signal quality when compared to additive noise of the same power. As such, there may be a need to improved the signal level of the detected hologram and improve the signal to noise ratio.